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Commentary
Atypical Enteropathogenic
Escherichia coli: Typical Pathogens?
James P. Nataro*
*University of Maryland School of Medicine, Baltimore, Maryland, USA
Suggested
citation for this article
Escherichia coli is both the most abundant facultative commensal
of the human gastrointestinal tract and the most common bacterial cause
of human diarrhea (1). However, precise recognition
of E. coli pathotypes remains problematic. Enteropathogenic E.
coli (EPEC), classically associated with outbreaks of infant diarrhea,
harbors distinctive chromosomal (the locus of enterocyte effacement, or
LEE island) and plasmidborne (residing on the EPEC adherence factor, or
EAF, plasmid) virulence factors, which are linked by common gene regulators
(1). At the Second International Conference on
EPEC, held in São Paulo, Brazil, in 1995, the foremost authorities in
the field proclaimed the global importance of such "typical"
EPEC (tEPEC) but pondered the clinical relevance of strains carrying only
the LEE island (dubbed at that conference "atypical EPEC," or
aEPEC) (2). Had aEPEC lost the EAF plasmid? Had
it incidentally acquired only fragments or incomplete packages of virulence-associated
genes? Or were some aEPEC true pathogens of humans or animals?
In this issue of Emerging Infectious Diseases, Nguyen et al. propose
a distinct role for aEPEC in human infection (3).
Previously, these investigators reported a high prevalence of aEPEC among
pediatric diarrhea patients in Melbourne, including both infants and older
children (in contrast to the strong tendency for infants to be infected
with tEPEC) (4). Now these authors show that,
in contrast to patients infected with other pathogens, patients infected
with aEPEC are far more likely to experience diarrhea past 14 days, the
point long recognized as a clinical watershed that heralds increased risk
for illness and death. aEPEC's prevalence among diarrhea patients, the
pathogen's strong association with diarrheal symptoms, and the infection's
distinctively persistent nature argue for a high disease burden in Melbourne.
Although the authors define aEPEC strictly on the basis of positivity
for the LEE eae gene and failure to amplify a bfpA pilin
gene (not assessing additional plasmid loci), the absence of tEPEC serotypes
and the occurrence of disease in children older than infants suggest that
these are indeed aEPEC.
This communication also illustrates 2 principles that should be recognized
more generally with regard to diarrheogenic E. coli: 1) one can
authoritatively implicate a particular strain as a pathogen by (only)
outbreak implication or by volunteer studies, but one cannot definitively
prove by any data that a putative pathotype is not a human pathogen; and
2) the implication of pathogenicity for 1 strain is not sufficient to
implicate any similar strain as a pathogen. Thus, once definitive evidence
is presented that a particular strain is pathogenic, the challenge is
to determine the genotypic or phenotypic fingerprints that permit extrapolation
to other strains. This obstacle has long proved daunting for the enteroaggregative
pathotype (shown to be pathogenic in both volunteer studies and outbreaks),
and it may prove similarly frustrating for aEPEC.
Diarrheal epidemiology studies today are able to identify a likely etiologic
agent for most patients, in contrast to studies of 20 years ago, because
of improved diagnostic tests and the identification of new pathogens,
but the task is not finished. Additional studies are needed in many places
to refine our understanding of putative and emerging pathogens and to
determine their full epidemiologic roles.
References
- Nataro JP, Kaper JB. Diarrheagenic
Escherichia coli. Clin Microbiol Rev. 1998;11:142–201.
- Kaper JB. Defining EPEC. Rev Microbiol Sao Paulo. 1996;27(Suppl1):130–3.
- Nguyen RN, Taylor LS, Tauschek M, Robins-Browne RM. Atypical enteropathogenic
Escherichia coli infection and prolonged diarrhea. Emerg Infect
Dis. 2006;12:597–603.
- Robins-Browne RM, Bordun A-M, Tauschek M, Bennett-Wood V, Russell
J, Oppedisano F, et al. Escherichia
coli and community-acquired gastroenteritis, Melbourne, Australia.
Emerg Infect Dis. 2004;10:1797–805.
Suggested citation
for this article:
Nataro JP. Atypical
enteropathogenic Escherichia coli: typical pathogens? [commentary].
Emerg Infect Dis [serial on the Internet]. 2006 Apr [date cited].
Available from http://www.cdc.gov/ncidod/EID/vol12no04/06-0125.htm
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